Composite separator for polymer electrolyte membrane fuel cell and method for manufacturing the same

ABSTRACT

The present invention provides a composite separator for a polymer electrolyte membrane fuel cell (PEMFC) and a method for manufacturing the same, in which a graphite foil prepared by compressing expanded graphite is stacked on a carbon fiber-reinforced composite prepreg or a mixed solution prepared by mixing graphite flake and powder with a resin solvent is applied to the cured composite prepreg such that a graphite layer is integrally molded on the outermost end of the separator. 
     For this purpose, the present invention provides a method for manufacturing a composite separator for a polymer electrolyte membrane fuel cell, the method including: preparing a prepreg as a continuous carbon fiber-reinforced composite and a graphite foil; allowing the cut prepreg and graphite foil to pass through a stacking/compression roller to be compressed; allowing the prepreg in which the graphite foil is integrally stacked to be heated and pressed by a hot press such that hydrogen, air, and coolant flow fields are formed or to pass through a hot roller to be formed into a separator; removing unnecessary portions from the heated and pressed separator using a trim cutter; and post-curing the thus formed separator, wherein the graphite foil may be stacked on the prepreg as the continuous carbon fiber-reinforced composite such that a graphite layer is integrally formed with the prepreg.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims under 35 U.S.C. §119(a) the benefit of KoreanPatent Application No. 10-2009-0116606 filed Nov. 30, 2009, the entirecontents of which are incorporated herein by reference.

BACKGROUND

(a) Technical Field

The present disclosure relates to a separator for a polymer electrolytemembrane fuel cell (PEMFC). More particularly, it relates to a compositeseparator for a PEMFC and a method for manufacturing the same, in whicha graphite foil prepared by compressing expanded graphite is stacked ona carbon fiber-reinforced composite prepreg or a mixed solution preparedby mixing graphite flake and powder with a resin solvent is applied tothe cured composite prepreg such that a graphite layer is integrallymolded on the outermost end of the separator.

(b) Background Art

In general, a polymer electrolyte membrane fuel cell (PEMFC) is a devicethat generates electricity with water produced by an electrochemicalreaction between hydrogen and oxygen. The PEMFC has various advantagessuch as high energy efficiency, high current density, high powerdensity, short start-up time, and rapid response to a load change ascompared to the other types of fuel cells. For these reasons, it can beused in various applications such as a power source for zero-emissionvehicles, an independent power plant, a portable power source, amilitary power source, etc.

The configuration of a fuel cell stack will be briefly described withreference to FIG. 1 below.

In a typical fuel cell stack, a membrane-electrode assembly (MEA) ispositioned in the center of each unit cell of the fuel cell stack. TheMEA comprises a solid polymer electrolyte membrane 60, through whichhydrogen ions (protons) are transported, and catalyst layers including acathode 61 and an anode 61, which are coated on both surfaces of theelectrolyte membrane 60 to allow hydrogen and oxygen to react with eachother.

Moreover, a gas diffusion layer (GDL) 40 and a gasket 41 aresequentially stacked on the outside of the electrolyte membrane 10,i.e., on the surface where the cathode and the anode are positioned. Aseparator (also called a bipolar plate) 30 including flow fields,through which fuel is supplied and water generated by a reaction isdischarged, is positioned on the outside of the GDL 40. And, an endplate 50 for supporting the above-described elements is connected to theoutermost end.

Therefore, an oxidation reaction of hydrogen occurs at the anode of afuel cell to produce hydrogen ions (protons) and electrons, and theproduced hydrogen ions and electrons are transmitted to the cathodethrough the electrolyte membrane and the separator, respectively. At thecathode, the hydrogen ions and electrons transmitted from the anodethrough the electrolyte membrane and the separator react with oxygen inair to produce water. Here, electrical energy is generated by the flowof the electrons through an external conducting wire due to the transferof the hydrogen ions.

In the above-described fuel cell stack, the separator separates therespective unit cells of the fuel cell and, at the same time, serves asa current path between the unit cells, and the flow fields formed in theseparator serve as paths through which hydrogen and oxygen are suppliedand water produced by the reaction is discharged.

Since the water produced by the reaction inhibits the chemical reactionoccurring in the electrolyte membrane of the fuel cell, the water shouldbe rapidly discharged to the outside, and therefore the separatormaterial may have high surface energy such that the water is rapidlyspread on the surface of the separator (hydrophilicity) or may have lowsurface energy such that the water rolls down the surface of theseparator (hydrophobicity).

Therefore, it is necessary to minimize the electrical contact resistancebetween the separators and, at the same time, maximize thehydrophilicity or hydrophobicity of the flow fields to allow the productwater to smoothly flow.

Conventionally, the separator is formed of graphite, thin stainlesssteel, or a composite material in which expanded carbon particles orgraphite particles are mixed with a polymer matrix. However, recently,an attempt to prepare a composite separator using continuous carbonfibers has been made.

Although the electrical resistance of stainless steel is significantlylower than that of graphite (see Table 1), the electrical contactresistance of graphite is measured lower than that of stainless steel,since the electrical contact resistance is related to the contact areaand pressure and the hardness of the material.

Moreover, although the graphite satisfies the conditions of theseparator in its electrical and chemical requirements, it is vulnerableto impact and is hard to process. Therefore, a research aimed atdeveloping a continuous carbon fiber composite separator which satisfiesthe electrical, chemical, and mechanical requirements has continued toprogress. Further, in order to reduce the contact resistance between theunit cells, and to efficiently discharge water produced by the reaction,a method for manufacturing a separator capable of controlling thesurface energy of the continuous carbon fiber composite separator isrequired.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY OF THE DISCLOSURE

In one aspect, the present invention provides a method for manufacturinga composite separator for a polymer electrolyte membrane fuel cell, themethod including: preparing a prepreg as a continuous carbonfiber-reinforced composite and a graphite foil; allowing the cut prepregand graphite foil to pass through a stacking/compression roller to becompressed; allowing the prepreg in which the graphite foil isintegrally stacked to be heated and pressed by a hot press such thathydrogen, air, and coolant flow fields are formed or to pass through ahot roller to be formed into a separator; removing unnecessary portionsfrom the heated and pressed separator using a trim cutter; andpost-curing the thus formed separator, wherein the graphite foil isstacked on the prepreg as the continuous carbon fiber-reinforcedcomposite such that a graphite layer is integrally formed with theprepreg.

In another aspect, the present invention provides a method formanufacturing a composite separator for a polymer electrolyte membranefuel cell, the method including: preparing a prepreg as a continuouscarbon fiber-reinforced composite; preparing a mixed solution by mixingat least one selected from the group consisting of graphite flake,graphite powder, and carbon black nanoparticles with a resin solvent;applying the mixed solution to both surface of the prepreg while theprepreg passes through a cutting roller having a cutter to be cut in thelongitudinal direction of the separator or applying the mixed solutionto both surface of the prepreg while the prepreg cut by the cuttingroller with the cutter passes through a stacking/compression roller;allowing the prepreg to which the mixed solution is applied to be heatedand pressed by a hot press such that hydrogen, air, and coolant flowfields are formed or to pass through a hot roller to be formed into aseparator; removing unnecessary portions from the heated and pressedseparator using a trim cutter; and post-curing the thus formedseparator, wherein the mixed solution is applied to the prepreg as thecontinuous carbon fiber-reinforced composite such that a graphite layeris integrally formed with the prepreg.

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g. fuels derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example bothgasoline-powered and electric-powered vehicles.

The above and other aspects and features of the invention are discussedinfra.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now bedescribed in detail with reference to certain exemplary embodimentsthereof illustrated the accompanying drawings which are givenhereinbelow by way of illustration only, and thus are not limitative ofthe present invention, and wherein:

FIG. 1 is a schematic diagram showing the configuration of a fuel cellstack.

FIG. 2 is a process diagram showing a method for manufacturing acomposite separator for a PEMFC according to Example 1 of the presentinvention.

FIG. 3 is a process diagram showing a method for manufacturing acomposite separator for a PEMFC according to Example 2 of the presentinvention.

FIGS. 4 and 5 are schematic diagrams showing a graphite layer formed ona separator in accordance with the present invention.

FIG. 6 is an electron microscope image of a graphite layer formed on aseparator in accordance with the present invention.

FIG. 7 is a schematic diagram showing an electrical resistance test on aseparator specimen in accordance with the present invention.

FIG. 8 is a graph showing the results of the electrical resistance teston a separator specimen in accordance with the present invention and aconventional separator specimen.

Reference numerals set forth in the Drawings includes reference to thefollowing elements as further discussed below:

 10: continuous carbon fiber prepreg 11: graphite foil  12: roll 15:graphite flake and powder  16: cutter 18: cutting roller  20:stacking/compression roller 22: hot press  24: positive and negativeflow field patterns  26: trim cutter 28: hot roller 200: resin solvent

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of theinvention. The specific design features of the present invention asdisclosed herein, including, for example, specific dimensions,orientations, locations, and shapes will be determined in part by theparticular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent partsof the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Hereinafter reference will now be made in detail to various embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings and described below. While the invention will bedescribed in conjunction with exemplary embodiments, it will beunderstood that present description is not intended to limit theinvention to those exemplary embodiments. On the contrary, the inventionis intended to cover not only the exemplary embodiments, but alsovarious alternatives, modifications, equivalents and other embodiments,which may be included within the spirit and scope of the invention asdefined by the appended claims.

For a better understanding of the present invention, the properties of acontinuous carbon fiber-reinforced composite and graphite used in themanufacturing of the separator of the present invention, and those ofother existing materials (carbon powder composite, metal (e.g.,stainless steel (SUS)) are shown in the following table 1:

TABLE 1 Carbon Carbon fiber powder Metal reinforced DOE PropertiesGraphite composite (SUS) composite reference Specific gravity 1.88 1.927.87 1.9 N/A (g/cm³) Thermal conductivity 100 08 to 20 16.3 48.4 to 60.640 (Raw material, W/mK) Contact resistance 15.6 20.2 75 20 to 30 25(@150 psig, mΩcm²) Coefficient of thermal 4.7 10 to 30 11 to 18 −0.12 to22   N/A expansion (10⁻⁶/K) Electrical conductivity 770 251 14,000 100to 125 Over 100 (S/cm) Flexural strength 85 50 510 1,550 Over 34 MPa(MPa) Compressive strength 170 Satisfied Satisfied Satisfied Over 105MPa (MPa) Corrosion resistance Satisfied Satisfied Expensive SatisfiedOver 1 mA/cm² coating required

As shown in Table 1, the continuous carbon fiber-reinforced compositeused in the present invention can contribute to a reduction in weightwith a specific gravity of about ¼ of the metal (SUS), reduce thethickness of the overall separator, which increases the reaction areaper unit volume to increase the power density, and satisfy thecompressive strength and the corrosion resistance. Moreover, thegraphite forming the outermost end of the separator has a high thermalconductivity, a significantly low contact resistance, and a high surfaceenergy (hydrophilicity) to quickly discharge water produced by areaction, which reduces the current loss in the fuel cell stack, thussignificantly increasing the efficiency of the fuel cell.

Therefore, the separator in accordance with the present invention can beeasily manufactured by stacking a graphite foil prepared by compressingexpanded graphite on a carbon fiber-reinforced composite prepreg orapplying a mixed solution prepared by mixing graphite flake and powderwith a resin solvent to the cured composite prepreg by a continuouslyprocess for mass production.

As discussed above, in one aspect, the present invention provides amethod for manufacturing a composite separator for a polymer electrolytemembrane fuel cell, the method comprising: preparing a prepreg as acontinuous carbon fiber-reinforced composite and a graphite foil;allowing the cut prepreg and graphite foil to pass through astacking/compression roller to be compressed; allowing the prepreg inwhich the graphite foil is integrally stacked to be heated and pressedby a hot press such that hydrogen, air, and coolant flow fields areformed or to pass through a hot roller to be formed into a separator;removing unnecessary portions from the heated and pressed separatorusing a trim cutter; and post-curing the thus formed separator.

In embodiments, a prepreg as a continuous carbon fiber-reinforcedcomposite and a graphite foil are prepared. The prepreg and the graphitefoil are cut in a longitudinal direction of the separator by a cuttingroller having a cutter. The cut prepreg and graphite foil are passedthrough a stacking/compression roller to be compressed. The stackedprepreg in which and the graphite foil are hot-pressed by a hot press ora hot roller such that hydrogen, air, and coolant flow fields areformed. Unnecessary portions from the hot-pressed stack of the prepregand the graphite foil are trimmed using a trim cutter to be formed intoa separator. The thus-formed separator is post-cured.

Preferably, the step of cutting may comprise a step of forming a commondistribution manifold and assembly holes.

Suitably, before the step of compressing, there may be provided with astep of stacking the cut prepreg and the cut graphite foil in such afashion that at least one of the prepreg is positioned between thegraphite foils.

Preferably, the step of compressing may be conducted at a temperaturerange between about 40 degree C. and about 80 degree C.

Preferably, the step of hot-pressing may be conducted at a temperaturerange between about 150 degree C. and about 250 degree C.

Preferably, the step of post-curing may be conducted in an autoclave forabout two hours at temperature of about 125 degree C.

As discussed above, in another aspect, the present invention provides amethod for manufacturing a composite separator for a polymer electrolytemembrane fuel cell, the method comprising: preparing a prepreg as acontinuous carbon fiber-reinforced composite; preparing a mixed solutionby mixing at least one selected from the group consisting of graphiteflake, graphite powder, and carbon black nanoparticles with a resinsolvent; applying the mixed solution to both surface of the prepregwhile the prepreg passes through a cutting roller having a cutter to becut in the longitudinal direction of the separator or applying the mixedsolution to both surface of the prepreg while the prepreg cut by thecutting roller having the cutter passes through a stacking/compressionroller; allowing the prepreg to which the mixed solution is applied tobe heated and pressed by a hot press such that hydrogen, air, andcoolant flow fields are formed or to pass through a hot roller to beformed into a separator; removing unnecessary portions from the heatedand pressed separator using a trim cutter; and post-curing the thusformed separator.

Examples for manufacturing separator in accordance with the presentinvention will be described below, but the present invention is notlimited to the following Examples.

Example 1

A continuous carbon fiber prepreg and a graphite foil were molded by ahot pressing process to manufacture a separator of the presentinvention.

FIG. 2 is a process diagram showing the method used for manufacturing acomposite separator according to Example 1.

First, a semi-cured sheet-like prepreg 10 as a raw material of acontinuous carbon fiber-reinforced composite having a length of severalmeters to several tens of meters, in which continuous carbon fibershaving a diameter of about 7 μm were surrounded by a thermosettingpolymer binder, and a continuous graphite foil 11 were wound on a roll12, respectively.

The prepreg 10 as the raw material of the continuous carbonfiber-reinforced composite and the graphite foil 11 were then passedthrough a plurality of cutting rollers 18 in the form of a long rolleach including a cutter 16 provided on the surface thereof such that theprepreg 10 and the graphite foil 11 were cut in the longitudinaldirection of the separator and, at the same time, a common distributionmanifold and an assembly hole were formed on the prepreg 10.

Next, the prepreg 10 cut in the longitudinal direction of the separatorand the graphite foil 11 stacked on both surfaces of the prepreg 10 weresimultaneously passed through a stacking/compression roller 20 or aplurality of prepregs 10 cut in the longitudinal direction of theseparator at regular intervals were arranged in a zigzag manner, i.e.,arranged in parallel and perpendicular to the carbon fibers such as0°/90°/0°, and the graphite foil 11 was stacked on both surfaces of theprepreg 10 and then passed through the stacking/compression roller 20.Here, the stacking/compression roller 20 was equipped with a separateheating means (not shown) such that the prepreg 10 and the graphitefoils 11 were pressed and adhered at a temperature of 40 to 80° C. whilepassing through the stacking/compression roller 20. The reason for thiswas that if the prepreg 10 and the graphite foils 11 are pressed at atemperature of lower than 40° C., the adhesive strength between theprepreg 10 and the graphite foils 11 may be reduced, whereas, if theyare pressed at a temperature of higher than 80° C., they may be cured.Therefore, it is preferable that the prepreg 10 and the graphite foils11 be pressed in the temperature range of 40 to 80° C.

Next, the continuous carbon fiber prepreg 10 stacked in a single ormulti-layer together with the graphite foils 11 was placed on a hotpress 22 to be press-molded. Here, the molding temperature of the hotpress 22 with respect to the prepreg 10 was maintained at a temperatureof 150 to 250° C. The reason for this was that if the moldingtemperature is lower than 150° C., the moldability may be deteriorated,whereas, if it is higher than 250° C., the resin of the prepreg 10 maybe cured.

As shown in FIG. 4, a small amount of surplus resin of the continuouscarbon fiber prepreg 10 was impregnated into the graphite foil 11stacked on the outermost end of the continuous carbon fiber prepreg 10to be mechanically bonded thereto, and thereby a graphite layer wasformed on the outermost end of the separator by the graphite foil 11.Meanwhile, the graphite foil 11 was prepared by compressing expandedgraphite, and thus, upon completion of the molding process by the hotpress 22, it was possible to easily separate the separator molded by thehot press 22 without the use of a release paper. Moreover, positive andnegative flow field patterns 24 for forming hydrogen, air, and coolantflow fields were formed on upper and lower platens of the hot press 22.Therefore, the hydrogen, air, and coolant flow fields were formed on theprepreg 10 on which the graphite foils 11 were stacked by the pressmolding of the hot press 22.

Next, the heated and press-molded prepreg 10 including the graphitefoils 11 was subjected to a trimming process of removing unnecessaryportions from the separator using a trim cutter 26. The trimming processwas performed within a minimum period of time such that the heated andpress-molded prepreg 10 had a curing degree that could allow the prepreg10 to maintain its shape.

Finally, a post-curing process, in which about 400 separators wereplaced in an autoclave at the same time to be heat-treated at about 125°C. for about 2 hours, was performed to allow the separators to befinally cured.

Example 2

In this example, a continuous carbon fiber-reinforced composite and agraphite foil were molded by a hot rolling process to manufacture aseparator of the present invention.

FIG. 3 is a process diagram showing the method used for manufacturing acomposite separator according to Example 2.

First, in the same manner as Example 1, a semi-cured sheet-like prepreg10 as a raw material of a continuous carbon fiber-reinforced compositehaving a length of several meters to several tens of meters, in whichcontinuous carbon fibers having a diameter of about 7 μm are surroundedby a thermosetting polymer binder, and a continuous graphite foil 11were wound on a roll 12, respectively. The prepreg 10 as the rawmaterial of the continuous carbon fiber-reinforced composite and thegraphite foil 11 were passed through a plurality of cutting rollers 18in the form of a long roll each including a cutter 16 provided on thesurface thereof such that the prepreg 10 and the graphite foil 11 werecut in the longitudinal direction of the separator and, at the sametime, a common distribution manifold and an assembly hole were formed onthe prepreg 10.

Next, the prepreg 10 cut in the longitudinal direction of the separatorand the graphite foil 11 stacked on both surfaces of the prepreg 10 weresimultaneously passed through a stacking/compression roller 20 or aplurality of prepregs 10 cut in the longitudinal direction of theseparator at regular intervals were arranged in a zigzag manner, i.e.,arranged in parallel and perpendicular to the carbon fibers such as0°/90°/0°, and the graphite foil 11 was stacked on both surfaces of theprepreg 10 and then passed through the stacking/compression roller 20.Here, the stacking/compression roller 20 was equipped with a separateheating means (not shown) such that the prepreg 10 and the graphitefoils 11 were pressed and adhered at a temperature of 40 to 80° C. whilepassing through the stacking/compression roller 20. The reason for thiswas that if the prepreg 10 and the graphite foils 11 are pressed at atemperature of lower than 40° C., the adhesive strength between theprepreg 10 and the graphite foils 11 may be reduced, whereas, if theyare pressed at a temperature of higher than 80° C., they may be cured.Therefore, it is preferable that the prepreg 10 and the graphite foils11 be pressed in the temperature range of 40 to 80° C.

Next, the continuous carbon fiber prepreg 10 stacked in a single ormulti-layer together with the graphite foils 11 was placed on a hotroller 28 to be press-molded, heated, and partially cured. Here,positive and negative flow field patterns 24 for forming hydrogen, air,and coolant flow fields were formed on surface of the hot roller 28.Accordingly, the hydrogen, air, and coolant flow fields were formed onthe prepreg 10 while the prepreg 10 and the graphite foil 11 were passedthrough the hot roller 28 to be press-molded. A small amount of surplusresin of the continuous carbon fiber prepreg 10 was impregnated into thegraphite foil 11 in the same manner as Example 1, and thereby a graphitelayer was formed on the outermost end of the separator. Meanwhile, theheating temperature of the hot roller 28 with respect to the prepreg 10was maintained at a temperature of 150 to 250° C., although the heatingtemperature was different depending on the types of resin of the prepreg10.

Next, the heated and press-molded prepreg 10 including the graphitefoils 11 was subjected to a trimming process of removing unnecessaryportions from the separator using a trim cutter 26.

Finally, a post-curing process, in which about 400 separators wereplaced in an autoclave at the same time to be heat-treated at about 125°C. for about 2 hours, was performed to allow the separators to befinally cured.

Unlike the conventional method in which the powder-based composite ismolded into the separator using a mold, according to the presentinvention it is possible to easily manufacture the separator having thegraphite layer by the continuous hot pressing or hot rolling process formass production, in which the graphite foil is stacked on the continuouscarbon fiber-reinforced composite prepreg or the graphite in the form ofpowder is sprayed onto the continuous carbon fiber-reinforced compositeprepreg.

As can be seen from the above-described Examples 1 and 2, it is possibleto form the graphite layer by stacking the graphite foil on thecontinuous carbon fiber-reinforced composite prepreg to be molded by ahot press or hot roller.

Example 3

In this example, a mixed solution prepared by mixing at least oneselected from the group consisting of graphite flake, graphite powder,and carbon black nanoparticles with a resin solvent was used.

For example, as shown in FIG. 5, it is possible to form the graphitelayer by mixing graphite flake and powder 15 or carbon black with aresin solvent 200 such as methylethylketone (MEK) or acetone andspraying the mixture onto the continuous carbon fiber-reinforcedcomposite prepreg. That is, a process of mixing graphite flake andpowder having a particle size of 3 to 500 μm (5,000 mesh−35 mesh) orcarbon black as nanoparticles with a resin solvent and spraying themixture on the prepreg may be performed between the process of cuttingthe prepreg and the process of passing the prepreg through thestacking/compression roller. As shown in FIG. 5, the graphite flake andpowder 15 are mixed with the resin solvent 200 and the mixture issprayed onto the outermost surface of the stacked prepreg.

Therefore, the graphite particles sprayed with the solvent remain on thesurface of the composite to form the graphite layer, thus reducing theelectrical contact resistance, increasing the hydrophilicity of theseparator surface, and further serving as a release paper used duringthe molding process of the separator.

Test Example 1

It was determined whether the graphite foil was integrally stacked onthe continuous carbon fiber prepreg of the separators manufacturedaccording to Examples 1 and 2 by photographing the cross sections of theseparator specimens using an electron microscope, and the results areshown in FIG. 6.

As shown in FIG. 6, it can be seen that the graphite layer wasintegrally formed on the surface of the continuous carbon fiber prepregand a part of the graphite layer was impregnated into the continuouscarbon fiber prepreg with respect to the boundary between the continuouscarbon fiber prepreg and the graphite layer.

Test Example 2

The electrical resistances of the specimens used in Test Example 1, theseparator specimens in which the graphite foil was stacked on thecontinuous carbon fiber prepreg according to Examples 1 and 2, theseparator specimen in which carbon black particles were applied to thecontinuous carbon fiber prepreg according to Example 3, and a specimenwith no graphite layer according to Comparative Example, were measuredin such a manner that a gas diffusion layer was disposed on a copperplate attached on the inner surface of each insulating plate and thespecimen was interposed between the gas diffusion layers and fixed at apredetermined clamping pressure as shown in FIG. 7.

Here, since the specimen according to the Comparative Example had nographite layer, its thickness was 0.67 t, and the specimens according toExamples 1 and 2 of the present invention having the graphite layer hadan increased thickness of 0.8 t.

The measured electrical resistances of the specimens according toExamples 1 and 2 of the present invention, the specimen in which carbonblack particles were applied to the continuous carbon fiber prepregaccording to Example 3, and the specimen with no graphite layeraccording to the Comparative Example are shown in the following table 2and the graphs of FIGS. 8A and 8B:

TABLE 2 Comparative Examples 1 and 2 Example 3 (mΩ) Pres- Example (mΩ)(mΩ) Composite Composite plate sure Composite plates (0.8 t) + Graphite(0.2 t) + Carbon (MPa) plates (0.67 t) foil on both sides black on bothsides 0.50 31.71 5.57 3.43 0.75 26.71 4.71 2.29 1.00 23.14 4.29 1.861.25 21.43 4.00 1.57 1.5 19.86 3.71 1.29 1.75 18.71 3.57 1.14 2.00 17.573.43 1.00

As shown in table 2 and FIG. 8, it can be seen that the total electricalresistances (R_(TOTAL)) measured from the specimens according toExamples 1 and 2 of the present invention was about 18% of that of thespecimen according to the Comparative Example, from which it can beunderstood that it is possible to significantly reduce the electricalcontact resistance by forming the graphite layer on the continuouscarbon fiber prepreg.

According to the present invention, the graphite layer is formed on theoutermost end of the continuous carbon fiber composite separator toreduce the electrical contact resistance and increase the hydrophilicityof the surface of the separator, thus efficiently removing waterproduced by the reaction from the fuel cell. That is, the graphite foilprepared by compressing expanded graphite is stacked on the carbonfiber-reinforced composite prepreg or the mixed solution prepared bymixing graphite flake and powder with a resin solvent is applied to thecured composite prepreg such that the graphite layer is integrallyformed on the outermost end of the separator, thus reducing theelectrical contact resistance of the separator and increasing thehydrophilicity of the surface of the separator. As a result, it ispossible to efficiently discharge water produced by the reaction,thereby reducing the energy loss.

The invention has been described in detail with reference to preferredembodiments thereof. However, it will be appreciated by those skilled inthe art that changes may be made in these embodiments without departingfrom the principles and spirit of the invention, the scope of which isdefined in the appended claims and their equivalents.

1-3. (canceled)
 4. A method for manufacturing a composite separator fora polymer electrolyte membrane fuel cell, the method comprising:preparing a prepreg as a continuous carbon fiber-reinforced composite;preparing a mixed solution by mixing at least one selected from thegroup consisting of graphite flake, graphite powder, and carbon blacknanoparticles with a resin solvent; applying the mixed solution to bothsurface of the prepreg while the prepreg passes through a cutting meansto be cut in the longitudinal direction of the separator or applying themixed solution to both surface of the prepreg while the prepreg cut bythe cutting roller with the cutter passes through a stacking/compressionroller; allowing the prepreg to which the mixed solution is applied tobe heated and pressed by a hot press such that hydrogen, air, andcoolant flow fields are formed or to pass through a hot roller to beformed into a separator; removing unnecessary portions from the heatedand pressed separator using a trim cutter; and post-curing the thusformed separator, wherein the mixed solution is applied to the prepregas the continuous carbon fiber-reinforced composite such that a graphitelayer is integrally formed with the prepreg.
 5. The method of claim 4,wherein the mixed solution is prepared by mixing graphite flake orgraphite powder having a particle size of 3 to 500 μm (5,000 mesh−35mesh), or carbon black nanoparticles with a resin solvent and applied tothe prepreg. 6-7. (canceled)
 8. A separator for a fuel cell prepared bythe method of claim 4, wherein the graphite layer is integrally formedon the outer surface of the prepreg.
 9. The separator of claim 8,wherein a small amount of surplus resin of the prepreg is impregnatedinto the graphite layer formed by the graphite foil stacked on theoutermost end of the prepreg to be mechanically bonded thereto.